You need multiple boilers. One coal and one an alternate backup fuel. Size the coal boiler for providing the heat needed for all but the coldest days. Piping should be in a primary-secondary loop for automatic supplemental heat from the second boiler. Take your time and design the system right from the beginning. Buy this book.http://www.hydronicpros.com/publication ... .php?id=24
It comes with software that will help you design your system. If you can't afford everything now at least do the basics of the heat distribution system right from the beginning. You need to do a heat loss calculation on your home. The beginning chapters in the book tell you how.
If your house will also be A/C you need a Manual J analysis. This will give you heat loss and heat gain. Best left to a professional. You want "full Manual J" 8th edition analysis. If the guy doing it doesn't have a computer to do the calculations find someone that does.
Underground insulated PEX piping will work just fine. You will need an anti-freeze loop to heat your pool with heat exchangers unless you design a winter drain down system. Do not oversize the boiler just because of the pool Btu load. Your heating season efficiency and economy will suffer. Again buy the book!